This invention is directed to an improved oscillator circuit, and in particular to an oscillator circuit having a C-MOS inverter and a secondary inverter that is selectively coupled, in parallel, with the C-MOS inverter to reduce the impedance at the output of the C-MOS inverter and thereby increase the closed loop gain of the C-MOS inverter for a short interval of time.
Quartz crystal oscillator circuits, utilizing a quartz crystal vibrator as a high frequency time standard, have become well known in the art. Such quartz crystal vibrator oscillator circuits are able to produce highly stable, high frequency time standard signals and, hence, are particularly suitable for use in electronic timepieces for providing accurate timekeeping.
A quartz crystal oscillator circuit includes a quartz crystal vibrator, a C-MOS inverter having its output fed back through the quartz crystal vibrator to its input, and further feedback resistance elements and capacitor elements that stabilize the operation of the oscillator circuit. Accordingly, the supply voltage required to effect starting, stopping and stable operation of the oscillator circuit depends upon the closed loop gain of the oscillator circuit, which closed loop gain is determined in large measure by the impedance of the resistance and capacitance elements of the oscillator circuit, and the impedance at the output of the C-MOS inverter circuit.
Although it has been found that a reduction of the capacitance at the output of the C-MOS inverter will result in a reduction in the supply voltage necessary to effect oscillation of the circuit or to maintain same oscillating, a reduction in capacitance at the output of the C-MOS inverter often contribute, to less than completely stable operation. Similarly, it has also been found that an increase in the current supplied to the C-MOS inverter will also permit the quartz crystal oscillator circuit to commence operation at lower supply voltages and to continue operating at even lower supply voltages. Nevertheless, an increase in current consumption will rapidly dissipate the voltage source producing the supply voltage, which voltage source is usually a DC battery, when the oscillator circuit is utilized in an electronic wristwatch. Accordingly, an oscillator circuit wherein the supply voltage required to start same oscillating or to keep same oscillating is reduced, when a supply voltage is initially applied thereto, or when during operation the level of the supply voltage applied thereto is reduced, is desired.